The present invention relates to a refrigerant cycle apparatus in which a compressor, a radiator, a first pressure reducing device, an intermediate pressure receiver, a second pressure reducing device, and an evaporator are successively connected to one another in an annular form to constitute a refrigerant circuit and which is operated at a supercritical pressure on a high-pressure side.
In this type of conventional refrigerant cycle apparatus, for example, an air conditioner for cooling air in a room, a compressor, a radiator, a pressure reducing device, an evaporator and the like have heretofore been connected to one another in an annular form via piping to constitute a refrigerant cycle. Moreover, a refrigerant gas is sucked into a compression element of the compressor, and is compressed to form the refrigerant gas having high temperature and pressure. The gas is discharged, and flows into the radiator. In the radiator, a refrigerant radiates heat. The refrigerant which has flown out of the radiator is throttled by the pressure reducing device, and is supplied to the evaporator. In the evaporator, the refrigerant evaporates, and absorbs heat from its periphery to exert a cooling function and cool the inside of the room.
In recent years, in order to deal with a global environmental problem, also in this type of refrigerant cycle, an apparatus has been developed in which carbon dioxide (CO2) as a natural refrigerant is used as the refrigerant without using conventional chlorofluorocarbon and which is operated at a supercritical pressure on a high-pressure side (see Japanese Patent No. 2804527).
In this type of refrigerant cycle apparatus, when a temperature of a heat source for exchanging the heat with the refrigerant rises in the radiator, refrigerating effects remarkably decrease, and the pressure on the high-pressure side needs to be raised in order to compensate for the decrease. As a result, there has been a problem that a compressive power increases and performances degrade.
Moreover, since the carbon dioxide refrigerant has a less pressure loss as compared with another refrigerant, a pressure reducing degree has to be increased in the pressure reducing device. However, when a usual electronic expansion valve is used as such pressure reducing device, it is difficult to obtain desired throttling effects, and an appropriate control could not be performed.
On the other hand, when a capillary tube is used as the pressure reducing device, a length of the capillary tube has to be increased, or an inner diameter thereof has to be reduced in order to obtain desired pressure reducing effects. However, when the inner diameter is excessively reduced, the capillary tube is clogged with sludge, water content, or oil, and there is a possibility that a trouble is generated in refrigerant circulation. However, when the desired pressure reducing effects are to be obtained by a usual capillary tube having an inner diameter of 0.6 mm, the length of the tube becomes 20 m or more.